1. Field of Invention
This invention pertains to a plasma device with low thermal noise. More particularly, this invention pertains to a plasma device in which the device parameters are selected to provide an optimum noise level and signal-to-noise ratio.
2. Description of the Related Art
The signal-to-noise ratio (SNR) is the power ratio between a signal and noise. A high SNR is desirable because noise corrupts the signal, potentially reducing the bandwidth of the signal. Sources of noise include both external and internal sources. External sources include man-made noise, such as radio-frequency interference (RFI), and naturally generated noise, such as lightning and background or black body radiation. Internal sources include thermal noise generated by the various components of the circuit.
For electrical devices that conduct or are responsive to very low level signals, the SNR is one factor considered in the design of the device. Low signal levels are often encountered in sensor circuits and receiver front ends. For example, an antenna is a device that is responsive to electromagnetic waves, either transmitted or received. The performance of the antenna is affected by its SNR, with a high SNR being desirable. Increasing the SNR requires increasing the signal and/or decreasing the noise. In some cases, after the antenna design and configuration is optimized, a limit for signal sensitivity is reached where the internal sources of noise become significant to the SNR.
FIG. 1 illustrates a conventional metal wire loop antenna 100 that includes a pair of leads 104 providing an electrical connection to the wire loop 102. It is known to use metal antennas with various configurations, for example, dipole, folded dipole, beam, and loop. The antenna 100 is made of a conductive metal and its physical properties, such as length and arrangement, determine its electrical properties.
Nyquist's Theorem describes the noise power spectral density for commonly encountered noise, that is, for thermal noise for the type of electrical components in existence in 1928, when Nyquist developed the theorem. The Nyquist Theorem is applicable to metal based antennas and conductors and at low frequencies. The noise power spectral density for a metal antenna 100, H(f)metal, is given by the following equation:H(f)metal=4kTR  (1)
where H(f)metal is in units of volts2 per Hertz,
k is Boltzmann's constant in joules per Kelvin,
T is the temperature of the metal in degrees Kelvin, and
R is the resistance of the metal in Ohms.
As can be seen, reducing the temperature and or resistance reduces the noise power spectral density for a metal antenna 100. But, the physical properties of metal and the constraints of signal propagation do not often allow the temperature and resistance to be lowered. For example, refrigeration units add mass and bulk to systems, which is not desirable for airborne and other applications. An example of the impracticality of trying to reduce resistance is for systems that have a very high frequency such that the skin effect comes into play. The skin effect causes the effective resistance of a conductor to increase with the frequency of the current.